The present invention concerns a method for manufacturing quenched thin-walled metal hollow casings by blow-moulding.
A blow-moulding method for manufacturing metal hollow casings in one piece is previously known from SE 64 771, whereby the heated casing is expanded in a heated moulding by means of introducing a heated pressurized medium such as pressurized air, steam or other gaseous medium, so that the shape thus expands to match that of cavities arranged in the moulding. Since the shaping of the material takes place at high temperature, it is not only the actual formability of the material that increases, but the formation of the shape also occurs without the structure of the material being changed as long as this formation takes place at a temperature above the recrystallization temperature of the material. Because of this, tubular items can be produced with complex shapes in thin materials and with very good size accuracy.
Within the motor industry in particular, there has long been a wish to produce by a simple and cheap means and using thin-walled low alloy steel casing (less than 3 mm thick) quenched hollow casings in one piece as a replacement for the casings pressed and quenched to form suitable sheet billets, primarily flat and relatively thin billets, that, when joined together, form the load-bearing and protective frame components of a vehicle's body.
A common factor for the currently known tubular beam constructions is that they are expensive in manufacture due to the necessity of an extra manufacturing operation, namely the welding or gluing when the sheet billets are joined together. In addition, due to their joined-together design, the said beam constructions can in certain circumstances display construction weaknesses caused by notch effects and consequent problems of metal fatigue. In general, the stiffness performance is adversely affected in beam constructions manufactured according to the known technique.
As the manufacturing cost for the components that form part of a vehicle's safety cage, such as beams and their associated joining elements, has until now been very high in relation to the total cost of manufacture for the vehicle, it has not been possible to design these in an optimal way for the safety of those travelling in the vehicle. This all adds up to a major problem for the car industry, especially as the product life cycle for a vehicle has become shorter at the same time as concerns for safety have become more intense.
In addition to the said known technical difficulties in production associated with the manufacture of the said beam constructions mentioned above, the constructions have, due to the irregular shape at the location of the joints, sharp folds and cavities that increase the chance of corrosion and that are not easily accessible during treatment of the surfaces. In addition, the irregular form of known beam constructions increases their weight compared with the equivalent uniform item developed as one piece. Through the use of these known components, the weight of the car itself plus the possible payload will also increase, so that even the fuel consumption of the car will increase due to the greater engine performance that is thus required.
As mentioned above, such tubular beam constructions and similar elements have until now been manufactured by joining together sheet billets pressed in to suitable shapes whose moulding is previously known to employ that known as a pressing and quenching procedure, whereby both the moulding and the quenching of a sheet billet to produce the finished shape are performed in one and the same moulding tool. The main advantage of the said pressing and quenching procedure is that the item can be used directly in the quenched state without the requirement of subsequent tempering. It has proven to be particularly suitable to use carbonised manganese steel such as boron steel for this type of manufacturing process as this type of steel has very good quenching characteristics due to the addition of boron.
Such a manufacturing process is known, for example, through SE 435 527 whereby the starting material is a low alloy sheet billet, preferably a steel containing less than 0.4% carbon, silicon in an amount dependent on the method for manufacturing the steel but that is otherwise not critical, 0.5-2.0% manganese, a maximum of 0.05% phosphorous and a maximum of 0.05% sulphur, 0.1-0.5% chrome and/or 0.05-0.5% molybdenum, up to 0.1% titanium, 0.0005-0.01% boron, up to a maximum of totally 0.1% aluminium plus possible low concentrations of copper and nickel, possibly in amounts up to 0.2% each, whereby the material is heated to austenitising temperature, preferably 775-1000.degree. C. The sheet billet is then placed between two tools in a press and imparted with a significant change of shape by the tools being forced against each other by means of the press, and via rapid cooling of the tools to obtain an indirect rapid cooling of the billet, whereby this is quenched while remaining in the tool so that a martensitic and/or bainitic, preferably fine grain, structure is obtained.
It should be understood that this method is only applicable with flat, essentially plane shapes with a large surface area to lead away the heat and not, as in the case of the present invention concerning hollow casings, i.e. enclosed tubular shapes with surfaces that are relatively small and difficult to access to obtain a rapid cooling down of the billet by effectively leading away the heat.
Thus, the technique described in the said SE 64 771 named above does not refer to a method for achieving the sought-after kind of quenched, high strength hollow casing, in other words hollow casings of quenched steel formed in one piece. Neither does SE 435 527 give any guidance in this direction.